SEOUL (Yonhap) -- U.S. Assistant Secretary of State Christopher Hill made a three-day visit to Pyongyang last week hoping to make a breakthrough in North Korea's stalled denuclearization process, but the envoy gave no indication that his trip had produced any conclusive results.

Hill's trip came amid growing tension over the communist state's recent steps to reverse the disablement of its Yongbyon nuclear complex. Washington and Pyongyang have also been at odds over how to verify the North's June nuclear declaration.

Although no details have been disclosed, Seoul officials believe the talks yielded some results and that the North may have proposed a meeting of high-ranking military officials between Pyongyang and Washington to discuss the verification issue.

Upon returning to Seoul on Oct. 3, the chief U.S. nuclear envoy said he had "substantive" and "lengthy" talks with North Korean officials.

"There's been a lot of problems in the past couple of months regarding the six-party process, so we had a substantial review of activities," Hill said at a news conference after meeting with his South Korean counterpart, Kim Sook.

The six-party talks -- which also involve South Korea, China, Japan and Russia -- unraveled in August when Pyongyang said it was restarting its main nuclear facilities at Yongbyon following a dispute over Washington's failure to remove the North from its terrorism blacklist.

Hill said that among the North Korean officials he met during the trip were his counterpart Kim Kye-kwan and Lt. Gen. Ri Chan-bok, the chief military representative assigned to the inter-Korean border village of Panmunjom. Hill's meeting with Ri spawned media speculation that they might have discussed military issues, including a plan to formally end both the 1950-53 Korean War and Pyongyang's long-standing demand for mutual inspections of nuclear programs in South and North Korea.

The trip was Hill's third to North Korea for nuclear discussions, but was his first involving a meeting with a North Korean military official. Hill's trip to North Korea, originally scheduled to end on Oct. 2, was extended by an additional day.

Following the news conference, a South Korean diplomatic source hinted that a face-saving compromise might have been made during Hill's trip in which North Korea might come clean on its suspected uranium enrichment and proliferation and allow verification of its plutonium activities.

From Seoul, Hill flew to Beijing on Oct. 4 and met Chinese Vice Foreign Minister Wu Dawei, who chairs the six-way talks aimed at denuclearizing North Korea. The U.S. envoy returned to Washington after briefing his Chinese counterpart and the Russian ambassador to China on the negotiations.

Key negotiators have remained silent since Hill's talks, giving rise to speculation that the North may have made a significant proposal to the top U.S. nuclear negotiator. Some officials say North Korea may have proposed high-level talks between Washington and Pyongyang that would involve a comprehensive agenda, including military issues, nuclear disarmament and peace on the Korean Peninsula.

South Korean Foreign Minister Yu Myung-hwan on Oct. 7 hinted at progress in talks between North Korea and the United States on the verification of Pyongyang's nuclear program, saying both sides are showing flexibility.

"The U.S. is handling the issue with utmost flexibility, although there is no change in substantial content," Yu told lawmakers during an annual parliamentary inspection of his ministry's affairs.

Sung Kim, the U.S. special envoy to the six-party talks, is staying in Seoul to convey Washington's position to the South Korean government after an internal review of Hill's trip is concluded, the minister added.

The securing of a verification regime to check the authenticity of the North's recent nuclear declaration is the latest sticking point in the denuclearization process. The U.S. says a verification protocol that would allow soil sampling and access to key sites is a precondition for the removal of the North from its list of terrorism sponsoring nations under a 2007 aid-for-disarmament agreement. Pyongyang argues that Washington's verification plan is too strict and intrusive.

A South Korean government source said that North Korea is demanding "additional incentives" in return for accepting part of the U.S.'s terms. The North reportedly claims that the sites the U.S. wants to inspect include major military facilities, which must be relocated before the verification process begins.

But a pro-Pyongyang daily in Japan said on Oct. 6 that North Korea appears to have made to the United States a "bold and epochal" proposal that calls for a military and political resolution to break the current impasse in the multinational talks. Choson Sinbo, published by a pro-North Korean group in Japan, said Pyongyang delivered the "ultimatum" to the top U.S. nuclear envoy.

The newspaper said that if Washington fails to accept the proposal, North Korea would abandon the six-nation disarmament talks. It did not elaborate on the content of the proposal.

"The North Korean side appears to have suggested ways to peacefully resolve the nuclear dispute through the top U.S. negotiator to the six-party talks and issued an ultimatum," the daily said in a commentary. "The six-party talks, which were reactivated after North Korea's underground nuclear test in October 2006, may breakdown if the two sides fail to reach an agreement."

In such a case, North Korea would no longer adhere to the six-party framework and may try to reverse its nuclear disablement process to strengthen its negotiating power with the next U.S. government, the daily said. The fact that Pyongyang invited the U.S. nuclear envoy to the country, however, shows it does not want to waste diplomatic efforts made so far, the newspaper added.

South Korean officials have said that North Korea only issues invitations to foreigners if their application to travel there is approved.

The North announced in mid-August it would halt the disablement of its nuclear facilities in Yongbyon. Pyongyang told the International Atomic Energy Agency late last month that it was prepared to reload nuclear material into the plutonium reprocessing plant, where weapons-grade material would be extracted from spent fuel rods.

Diplomatic sources in Seoul say the U.S. and North Korea will likely hold additional talks this week, marking a pivotal point in the fate of the multinational talks. North Korea may demand cross-inspections on suspected nuclear sites in both South and North Korea, officials here said. Pyongyang has said its denuclearization depends on whether the U.S. can ensure that there are no U.S. nuclear weapons in South Korea. Seoul has maintained there are no U.S. nuclear weapons on its soil.

The U.S. military deployed tactical nuclear weapons to the peninsula decades ago, but pulled out them in the early 1990s.

Hill is expected to receive specific guidelines from senior staff in Washington on how to respond to the North's proposal, after briefing Secretary of State Condoleezza Rice and other parties in the six-nation talks. Sources say he may then send Kim, the U.S. special envoy to the six-party talks, to the Pyongyang for additional negotiations.

The U.S. may also hold consultations with South Korea ahead of its decision on the proposal, according to sources.

"The pending military issues between North Korea and the United States are unavoidably linked to discussions of peace on the Korean Peninsula. They are not only bilateral matters between the North and the United States, but also involve the six countries including South Korea and China," one source said.

Kim Sook, South Korea's chief nuclear negotiator, said on Oct. 3 that Seoul and Washington may hold high level talks to put the six-party negotiations back on track. But diplomatic sources have said Seoul would oppose any bilateral military talks between Washington and Pyongyang, as a discussion that excludes South Korea would damage Seoul's crucial interests.